The invention relates to a power transmission and bearing arrangement for a drum, the drum being meant for a rope which exerts a tangential force on the circumference of the drum, and the arrangement comprising the mounting of the drum in bearings to a frame structure and a drive arrangement for rotating the drum, the drive arrangement comprising at least one means which is in drive engagement with the drum surface. The drum may be meant for a lifting rope hanging from a lifting apparatus, although other similar applications are also possible.
The conventional way to arrange the power transmission and bearing arrangement of a drum is to provide the drum with a central shaft and bearings and to mount the drum to a frame structure, a drive arrangement being connected to the shaft. When this solution is applied to a lifting apparatus, the speed of the load suspended from the drum and, inversely, the torque in relation to the rate of rotation of the drive motor depend on the diameter of the drum and on the transmission ratio between the gear arranged between the motor and the drum and the cordage associated with the drum. Since economical manufacturing costs are to be aimed at, the drum diameter must be minimized to maintain a low transmission ratio in the gear. A small drum diameter, on the other hand, exposes the rope to wear.
Another alternative is to use the above described drum comprising the central shaft and mounted in the shaft bearings to the frame structure, only in such a way that the drive wheel of the drive arrangement is arranged to be in drive engagement with the drum surface, thus providing an additional transmission ratio in the power transmission of the drum, in addition to the transmission provide by the gear and the ropes. The magnitude of the transmission concerned is approximately equal to the magnitude of the portion by which the drum diameter reduces transmission, the drum diameter thus having no impact on the total transmission ratio. In this solution the drive wheel of the drum is usually placed on the circumference of the drum rearward from the spooling point of the rope by an amount equal to the pressure angle, thereby ensuring that the gear load and the rope force cancel each other as effectively as possible.
A problem with the above-mentioned solutions is that the method of arranging the drum bearing subjects the frame structure not only to rope force but also to a bending moment. In addition, when the drive is arranged directly on the drum surface, the frame structure must be made robust because the gear load cannot be completely cancelled and the accuracy of the shaft length must be guaranteed.
An object of the present invention is to remove the above drawbacks, starting from the type of arrangement described at the beginning. This is achieved with a method of the invention, characterized in that the means in drive engagement is substantially located, when seen in the circumferential direction of the drum surface, in an area on the other side of the drum surface between the means arranged to support and carry the drum surface, or substantially located on the other side of the drum surface at the means arranged to support and carry the drum surface, and that, seen from the end of the drum, the only point where the rope is unwound from the drum is located within the bearing area.
The invention is based on the idea that rope forces are led directly to the frame structures, thus providing light-weight and simple-structured frame parts. The lifting apparatus must naturally be attached to and/or suspended from the area where the rope unwound from the drum.
Due to the advantageous transmission ratio, a large drum diameter can be selected. This not only extends the service life of the rope but also allows a particular rope length (lifting height) to be obtained by a shorter drum, which reduces the drifting of the rope. A rope is said to drift when the spooling point of the rope on the drum shifts in the axial direction of the drum when the rope is wound on or off the drum. This also causes angles in the cordage that wear the rope and the rope grooves, and requires a wider suspension.
Other advantages of the invention are the possibility to make the structure of the lifting apparatus both compact and simple to manufacture and to assemble. In addition, different trolley arrangements are easy to attach to the frame, and the drive arrangement can be more freely selected and positioned than before.
Preferred embodiments of the invention are described in the independent claims.